1. Field of the Invention
The present invention relates to a process for the purification of a crude melt of at least one monomer by crystallization, in which the crude melt to be purified is subjected to a suspension crystallization, the monomer crystals produced during the suspension crystallization and suspended in residual melt are separated from the residual melt by using a first mechanical separation operation and, in at least one further crystallization stage, the residual melt remaining after the first mechanical separation operation and/or the monomer crystals which have been separated off and if necessary washed are further purified by crystallization after they have been melted.
2. Discussion of the Background
The term monomer in this document is intended to include chemical compounds which have at least one ethylenically unsaturated double bond.
Owing to the one or more ethylenically unsaturated double bonds, monomers form extremely reactive compounds which are used, inter alia, for the preparation of polymers. Typical examples of monomers are N-vinylpyrrolidone, acrylic acid, methacrylic acid and the alkyl esters of the abovementioned acids.
Usually, monomers are produced by chemical synthesis from suitable raw materials. Particularly because of impurities already present in the raw materials and owing to undesired secondary reactions, they are usually not present directly in pure form but are obtained as a component of mixtures from which they have to be isolated. Historically, in particular separation operations involving rectification or extraction and rectification are used as separation processes in this context (cf. for example EP-A 722926).
The disadvantage of these separation processes is that they have a high energy consumption since high reflux ratios and/or rectification columns having a large number of theoretical plates have to be employed.
As an alternative, the melt crystallization procedure for the preparation of pure monomers has been attracting increasing interest very recently (cf. for example EP-A 616998).
A crude melt of at least one monomer is obtained as a result of the synthesis and, if required, initial thermal and/or extraction separation stages. In this document, this is understood as meaning liquids which contain the one or more monomers and on cooling deposit as a first solid crystals of the one or more monomers which contain less of substances differing from the one or more monomers than does the crude melt itself.
This means that the term crude melt used here is not applicable when, for example, extracting agent or another component separates out as a first solid on cooling instead of the one or more monomers.
As a rule, the crude melts which are important according to the invention and comprise the one or more monomers contain small amounts of added polymerization inhibitors present in solution (cf. for example DE-A 19938841), which are intended to suppress an undesired free radical polymerization of the one or more monomers under the action of heat and/or light.
The one or more monomers can then be separated by crystallization from the above-defined crude melts of the one or more monomers in a manner known per se by the action of low temperatures, and a purified melt (in solid or in liquid form) of the one or more monomers can thus be prepared (cf. for example DE-A 19926082, WO 00/45928, WO 94/18166, DE-A 10026407, DE-A 10039025, DE-A 10003498 and DE-A 10003497).
Very generally, the contaminated crude melt is partially solidified by cooling. Depending on the phase equilibrium, the monomer crystals formed have a lower impurity content than the liquid residual melt remaining behind. The purely thermodynamically determined separation effect described above is reduced by the inclusion of liquid during the crystallization process and by the residual melts still adhering to the solid after a solid/liquid separation.
For achieving high purities and/or yields, a plurality of successive crystallization steps (also referred to as crystallization stages here) are often therefore required, i.e. the crystals obtained in a first crystallization stage, if necessary after they have been washed with a suitable solvent or with a melt of already purified crystals for removing residual melt, are remelted and are subjected to a further crystallization step, etc.
In order to render the yield economical, the residual melt obtained in the first crsytallization step is as a rule also subjected to at least one further crystallization step (in a further crystallization stage).
In general, different melt crystallization processes may be used for the purification of crude melts of at least one monomer by crystallization. In the layer crystallization processes, the one or more monomers are frozen out in the form of cohesive, firmly adhering layers.
The solid/liquid separation is effected by simply allowing the residual melt to flow away. The purified crystals can then be melted or can be dissolved in a desired solvent for further use.
In principle, a distinction is made between static and dynamic layer crystallization processes.
In the static process, the crude melt to be purified is introduced, for example, into tube-bundle heat exchangers or modified plate heat exchangers and then partially solidified by slow temperature reduction on the secondary side. After freezing, the residual melt is discharged and the crystal layer separated off is then melted as purified melt (pure melt), if necessary in stages. The heat and mass transfer to the separation or crystal layer surfaces is effected only by free convection.
Forced convection of the crude melt is typical of the dynamic layer crystallization of crude melts. This can be effected by pumping the crude melt through tubes with plug flow (e.g. German Laid-Open Application DOS 2,606,364), by feeding the crude melt as a falling film (e.g. EP-A 616998) or by passing inert gas into a tube filled with melt or by pulsation.
In the suspension crystallization process, a crystal suspension which contains the crystals separated off suspended in the residual melt is produced from the crude melt by the action of low temperatures. The solid crystals may be growing directly in suspension or may be deposited as a layer on a cooled wall, from which they are subsequently scraped off and resuspended in the residual melt. The separation of the deposited crystals from the residual melt is effected in the case of a crystal suspension usually by a mechanical separation operation (for example pressing, filtration, centrifuging and/or in wash columns).
The use of a combination of different crystallization methods is now frequently recommended for efficient separation of at least one monomer from the crude melts (cf. for example EP-A 616998).
For example, the crude melt can first be subjected to a suspension crystallization. The crystals which separate out thereby and are suspended in the residual melt are then separated from the residual melt by a mechanical separation operation and either are themselves the desired pure end product or are remelted, if necessary after they have been washed with, for example pure product melt (for example by resuspension therein) and are further purified in a further crystallization stage. This further crystallization stage may once again be a suspension crystallization but it may also be a layer crystallization. In a corresponding manner, the remaining residual melt can be further purified in a further crystallization stage. Although this can in principle likewise once again be a suspension crystallization, it may also be a layer crystallization.
Typical of all crystallization processes is that they have constrictions, i.e. regions with a narrow flow cross section. Thus, falling-film crystallizers usually contain, for example, internals which leave only a small flow cross section. The melt to be purified passes through this constriction only in the form of a thin film. Behind the constriction, this film is maintained and flows as a falling film down a cooled wall on which crystals are deposited during the flow process (cf. for example EP-B 218545).
In another manner, the separation of a crystal suspension into crystals and residual melt is effected almost always via cross sections through which only residual melt but not the suspended crystals can pass (for example via a two- or three-dimensional network of such cross sections in the case of filtration or in a screen centrifuge).
Usually, the crystallization processes for the purification of a crude melt of at least one monomer are carried out more or less continuously (or semicontinuously). A precondition for high space-time yields is that the narrow flow cross sections described are not blocked.
In contrast to the fractional layer crystallization process used in WO 0045928, there should be no problem with solid imported from the preceding suspension crystallization stage in the downstream crystallization stages (which are used for the further purification of the suspension crystals or for the further purification of the residual melt) in the process described in the preamble, in which it is essential initially to use a suspension crystallization stage. With the mechanical isolation of the suspension crystals, it is also intended to separate off other solids precipitated on reduction of the temperature, so that the remaining residual melt should be free from foreign solids.
However, the remelted suspension crystals should also be free from solids, and any concomitantly deposited foreign solids would in fact also go into solution again on melting.
When the process described in the preamble and intended for the purification of a crude melt of at least one monomer by crystallization was carried out in practice, such undesirable blockages occurred again and again (in particular in the case of acrylic acid, methacrylic acid and N-vinylpyrrolidone) in the crystallization stages for the further purification of a residual melt or of the suspension crystals, which crystallization stages follow the suspension crystallization stage to be used at the outset. This is particularly the case when the characteristic length of the constrictions is ≦5 mm. It is noteworthy that the material causing the blockage do not consist of crystals since the blockage generally could not be eliminated by heating above the melting point of the crystals.